A general view of the aircraft on the flight deck during a tour of the USS Nimitz aircraft carrier on patrol in the South China Sea in this file photo from May 23, 2013. The Nimitz and four other ships in its strike group moved into the Red Sea early September 1, 2013, U.S. defense officials said, describing the move as "prudent planning" in case the ships are needed for military action against Syria. (EDGAR SU/REUTERS)

A general view of the aircraft on the flight deck during a tour of the USS Nimitz aircraft carrier on patrol in the South China Sea in this file photo from May 23, 2013. The Nimitz and four other ships in its strike group moved into the Red Sea early September 1, 2013, U.S. defense officials said, describing the move as "prudent planning" in case the ships are needed for military action against Syria.(EDGAR SU/REUTERS)

NATO’s secretary-general says he is convinced that the Syrian regime used chemical weapons and that the world must respond firmly, as divisions widened among U.S. lawmakers over whether to approve President Barack Obama’s plans to strike.

Anders Fogh Rasmussen, the NATO political chief, said it will be up to individual nations to decide how they will act – but some analysts saw his strong call for action as opening the door to a potential role for NATO, a 28-nation alliance that includes Canada.

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The response to the alleged chemical weapons attack last month, which the Obama administration says was carried out by the Syrian government and killed more than 1,400 people, is now clearly entwined in a struggle for political backing in the West.

Mr. Obama met Monday with two interventionist Republican senators, John McCain and Lindsey Graham, to lobby for support for military action. Congress, and especially the House of Representatives, remains split. But Mr. McCain, saying he wants a strike that will damage the regime’s war-making capacity, warned that it would be “catastrophic” if Congress does not support the President.

So far only France has pledged to join in a U.S.-led strike. Prime Minister Jean-Marc Ayrault said Monday that its president does not need legislators’ approval, and that there will be no vote after a debate Wednesday in the National Assembly.

But the French government is trying to build political support for a strike at home and abroad. It released a nine-page intelligence report that came to the same conclusion as the White House, and says that the forces of Syrian President Bashar al-Assad regime had carried out a “massive and co-ordinated” chemical-weapons attack.

Mr. al-Assad, in an interview with the French newspaper Le Figaro published Monday, was defiant. He said Mr. Obama and French President François Hollande have been unable to provide proof of chemical-weapons use “even to their own people. He also warned an attack on Syria could trigger “regional war.”

Since British Prime Minster David Cameron lost a Commons vote on military action last week, advocates of a strike have been looking for political allies and they found one in Mr. Rasmussen. He said that he had been presented with “concrete information” on the alleged sarin gas attack on Aug. 21, and he is personally convinced a chemical attack took place and the Assad regime is responsible for it.

“It would send, I would say, a dangerous signal to dictators all over the world if we stand idly by and don’t react,” he told reporters at his monthly press conference in Brussels.

Mr. Rasmussen said it will be up to individual countries to decide how to respond, and he did not envisage further NATO involvement apart from ensuring the defence of Turkey, an alliance member. A short, targeted military operation against Syria would not require NATO’s “command and control system,” he said.

Still, some NATO-watchers said that Mr. Rasmussen’s comments are more likely to open the door to an alliance role, rather than close it.

“I think it is a transitional statement,” said Senator Hugh Segal, the chairman of the Atalantic Council of Canada, the civilian organization affiliated with NATO.

Mr. Rasmussen obviously cannot go too far ahead of the statements of NATO members, but his expression of support, along with his reference to convincing intelligence, sends signals that are helpful to the French and Americans, Sen. Segal said. But if major allies like the U.S., France and perhaps Turkey decide to strike Syria, and want to use NATO-country bases logistical support, “then I think his position would have to change,” he said.

That’s more likely to be true if those countries decide to launch a bigger operation than simply missile strikes from warships. They might want to use NATO’s Aviano air base in Italy, which would require Italy’s consent, and Mr. Rasmussen would start to face “a more serious situation in terms of major powers that are engaging.”

The Canadian government has said it is not planning to take part in any U.S.-led strikes, in part because it does not believe it has the weapons to help a missile campaign. Any NATO involvement could lead to more pressure on the members of the alliance to provide at least a small contribution. It would also be tricky for Mr. Cameron, although Sen. Segal noted that NATO has in the past approved missions even though not all members took a direct role.

Mr. Obama’s struggle for domestic political support in the U.S. might in fact lead to a bigger military operation than the limited strikes his administration portrayed as their planned response.

Several members of Congress, including hawkish Republicans in the House and Senate, like Mr. McCain and Mr. Graham, have said they will only support a strike if it will clearly degrade the Assad regime’s military capacity – arguing that an ineffective strike would be worse than none at all.